Structural similarity between the flagellar type III ATPase FliI and F₁-ATPase subunits

Construction of the bacterial flagellum in the cell exterior proceeds at its distal end by highly ordered self-assembly of many different component proteins, which are selectively exported through the central channel of the growing flagellum by the flagellar type III export apparatus. FliI is the ATPase of the export apparatus that drives the export process. Here we report the 2.4 Å resolution crystal structure of FliI in the ADP-bound form. FliI consists of three domains, and the whole structure shows extensive similarities to the α and β subunits of F₀F₁-ATPsynthase, a rotary motor that drives the chemical reaction of ATP synthesis. A hexamer model of FliI has been constructed based on the F₁-ATPase structure composed of the α₃β₃γ subunits. Although the regions that differ in conformation between FliI and the F₁-α/β subunits are all located on the outer surface of the hexamer ring, the main chain structures at the subunit interface and those surrounding the central channel of the ring are well conserved. These results imply an evolutionary relation between the flagellum and F₀F₁-ATPsynthase and a similarity in the mechanism between FliI and F₁-ATPase despite the apparently different functions of these proteins.